In recent years, harmful substances discharged in steps of paint application and plating in formation of members for vehicles have been even more strictly controlled in view of environmental issues, and thus acrylic decoration films and protection films have especially attracted attention as alternatives of such substances and for use in protecting the members. These protection films and decoration films are used by attaching to the surface of a substrate; therefore, transparency so as to allow printing and pattern applied onto the surface of the substrate to look brilliant, and flex cracking resistance upon insert or in-mold molding are demanded.
Moreover, since a film itself is printed, it requires in addition to chemical resistance, stain resistance against lactic acid components contained in human sweat and fats and oils on skin as interior members of vehicles are frequently brought into contact with human hand, and also against sunscreens, for example, Coppertone (registered trademark) used during summer season and in hot region as problems of deterioration of the substrate increase due to adhesion of such stains on the interior members.
In order to satisfy these quality requirements, a multilayer film produced by coextrusion molding of a methacrylic resin composition with a fluororesin that is superior in weather resistance and chemical resistance, as well as an acrylic resin film subjected to a surface hard coating treatment with a fluororesin have drawn strong interest in market. In particular, a method in which a film laminated with a vinylidene fluoride based resin which is melt moldable as a fluororesin laminated on an acrylic resin is decorated on the surface of a plastic formed article as a paint application alternative has attracted attention.
However, a vinylidene fluoride based resin is a crystalline resin, and has a high crystallization speed; therefore, to achieve satisfactory transparency is very difficult. In these regards, thinning of the film (see Patent Document 1), mixing of a vinylidene fluoride resin with a methacryl resin (see Patent Document 2), control of molding conditions including extrusion molding temperature, discharge rate and residence time in the extruder in film processing (see Patent Document 3) were proposed. However, required transparency cannot be easily achieved according to these methods.
Additionally, according to vinylidene fluoride resins, it is also difficult to achieve surface strength as desired for the aforementioned use.
Although a method for obtaining a vinylidene fluoride resin film that is superior in transparency and surface smoothness by sandwiching with metal rolls adjusted to have an appropriate temperature was studied (see Patent Document 4), increase of the steps may result in requirement for new facilities or remodeling of preexisting facilities, whereby disadvantages in economic aspects may occur. Therefore, a method which enables to produce a vinylidene fluoride resin film in a more convenient manner has been desired.